I disguise as demo and pyro because engi and sniper are EASILY the most common so they get spy checked a lot, but with engi and demo you dont get spy checked as often and you dont look suspicious due to changes movement speed or range, especially if it is a demoknight.

In highlander, you will rely on hiding spots instead of your disguise. Unless you are playing a complete noob team, any player will be able to spot you out in a matter of seconds, just by your walking pattern, and communications.

You don't use scout when entering the enemy base. You use a scout when not moving much in your own base, preferably near the back of a push. In a group, nobody pays attention to scouts. You can pick off a target or two from there.

The engineer (Lead character of this comic) tends to scream "SPY SAPPING MAH SENTRY" whenever a spy (those blue dudes in the background) uses an electronic sapper to disable constructs the engineer makes. hence he notices when someone "Saps" his wife